A volumetrically scanning type three-dimensional display is well known as a conventional display for reproducing a three-dimensional image in a space. This type three-dimensional display can reproduce a three-dimensional image that can be naturally focused to be viewed without using any special tool such as a stereoscopic glass.
The above-mentioned conventional volumetrically scanning type three-dimensional image display consists essentially of a laser light source, a modulator, an X-Y deflector, a control computer, an image-data memory and a moving flat screen.
Three-dimensional image data of an object desired to be displayed is first prepared. The moving flat screen moves from a initial position to a last position at a constant speed and instantly returns to the start position, and further repeats said cyclic movement. Cross-section images of the three-dimensional image, which corresponds to respective positions of the moving flat screen, are projected in turn to the moving flat screen by means of raster scanning with laser light under control of the control computer through the modulator and the X-Y deflector. At this time, a three-dimensional image is represented as an afterimage in human eyes in a space determined by multiplying the screen surface area by its moving stroke on the condition that the scanning speed of the laser beam and the moving speed of the moving flat screen are sufficiently synchronized with each other.
The above-mentioned conventional display allows the screen to move at a constant speed to a certain position and instantly return its initial position, i.e., makes the screen realize saw-tooth-like movement along a time base and quick return.
There is shown another example of a conventional three-dimensional display which uses a screen making a spiral movement that continuously varies its height in proportion to its rotation angle. Namely, the screen that rotates about a rotation axis can act like the moving flat screen that makes saw-tooth movement along the time base.
These conventional devices, however, encounter such a problem that laser light striking a point on the screen may reflect or scatter in all directions and may form a three-dimensional image that is a see-through semi-transparent image (with rear side seen therein).
Consequently, application of the conventional devices has been limited to, for example, a three-dimensional representation of previously sectioned images of a CT image, display for showing a relative position of an object in a space for an air-port control radar and the like.
Japanese Laid-Open Patent Publication No. 6-82612 discloses a three-dimensional image display that uses a diffraction grating array that is a substrate with arranged thereon a plurality of cells each consisting of diffraction grating. A three-dimensional image is represented by the diffraction array wherein the cells are divided into areas distributed each to pixels of each parallax image. This method is featured by that an image having parallax can be represented.
The plurality of cells consisting of diffraction grating is disposed on a flat substrate. Each cell is spatially divided into areas with near slope and distance of grating, which areas correspond to respective parallax images. The diffraction grating array is used as a basic device capable of displaying a three-dimensional image having parallax.
This three-dimensional image display using a diffraction grating array comprises a diffraction grating array, a liquid crystal display element being a spatial light-modulating element disposed on the rear surface of the diffraction grating array and a color filter layer disposed on the rear surface of the liquid crystal display element.
In this device, a small area of diffraction grating acts as follows:
The color filter layer selects a certain wave of white incident light, the liquid crystal display element selects transmission or no transmission of light and transmitted light arrives at the above-mentioned small area of the diffraction grating array.
The diffraction grating array made of light-transmission resin plate or the like allows light to pass being diffracted. The outgoing direction of the diffracted light is determined as a diffraction angle decided by a slope of the small area and the grating distance. The small area is seen bright in color of the selected wave when being viewed from the diffraction angle direction.
In conventional two- or three-dimensional display devices, hidden lines or surfaces are removed by using a Z-buffer method that is a memory for storing distance data in the depth direction, which data corresponds to respective pixels stored in the image data memory. The Z-buffer method is such that new pixel data to be outputted to the image data memory is compared to the data stored in the Z-buffer and the contents of the image data memory and the Z-buffer are updated only at a short distance from the view point.
As described above, a three-dimensional image obtained by the conventional three-coordinate scanning method is a semi-transparent image in which its rear side (hidden line or surface) appears. This principal drawback limits the field of its application to display three-dimensional image in predetermined sections (CT images) or relative positions of flying objects for an air-port control radar system and the like. Said drawback of an obtainable three-dimensional image may be eliminated by using a technique for removing hidden lines and hidden surfaces therefrom. The corrected three-dimensional image, however, has a single view point: it does not allow a plurality of observers to observe the image at the same time.
An image obtainable by the display disposed in Japanese Laid-Open Patent Publication No. 6-82612 is an incomplete three-dimensional image, i.e., a stereogram image like a lenticular stereogram image. Therefore, the three-dimensional image shows an object in a position mismatched with an image surface.
Furthermore, a three-dimensional image, from which hidden lines and surfaces have been removed by the Z-buffer method, has a limited view point and can not be observed by a plurality of observers at the same time.